Embodiments of the invention relate generally to a system and method for MR imaging and, more particularly, to a system and method for selectively and dynamically operating an array of RF receive coils in a transmit mode to generate a local RF field that adds up with the RF field generated by a whole body transmit coil, such that the transverse MR magnetization has the desired amplitude and phase.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a transverse RF magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In order to generate high quality images that minimize contrast and sensitivity variations, MRI applications require a uniform B1 field. Traditional quadrature-driven volume coils used for B1 field excitation provide limited uniformity of the field, especially as the strength/intensity of the magnetic field is increased (e.g., 3 T or 7 T magnetic fields). Therefore, the ability to generate a uniform B1 field is important for full realization of the potential of MRI applications utilizing a higher field strength.
Recently, several methods have been proposed in RF coil design to homogenize the B1 field. One such method for homogenizing the B1 field is parallel transmission. In existing MRI systems, parallel transmission corrects for the transmit B1 field inhomogeneity by having control over the amplitude and phase of individual transmit elements in a multi-channel transmit array coil, otherwise known as passive RF shimming. Parallel transmission further corrects for the transmit B1 field inhomogeneity by tailoring the magnetization by using spatially tailored RF pulses along with the gradients, otherwise known as active parallel transmit.
There are, however, several drawbacks to implementing existing methods of parallel transmission for purposes of homogenizing the B1 field. For example, as stated above, implementation of parallel transmission requires a multi-element transmit array coil, with the individual transmit elements needing to be well decoupled. For the multi-element transmit array, individual exciter boxes for fine control of the RF pulse waveform are needed, along with independent RF amplifiers for each element in the transmit array coil. Providing such a multi-element transmit array coil, and its associated elements, significantly increases the hardware cost for an MRI system.
It would therefore be desirable to have a system and method that provides a uniform B1 field without the need for a multi-element transmit array coil and associated components typically required for parallel transmission.